Jonathan studied at the NSW Conservatorium of Music with teachers such as Martin Wesley-Smith, Warren Burt, Graheme Hair, Vincent Plush and Don Banks. He received the first Don Banks memorial prize for composition in 1981. Since moving to Perth, Mustard has had a close collaborative relationship with composer/performer Lindsay Vickery. Vickery has premiered Mustard's work in Perth, The Hague, Los Angeles and Texas where he received an enthusiastic response to performances of improvisatory mixed media works such as Robosax I - The Strathfeild Massacre, Robosax III and Robosax IV - The Arsonist.
Jonathan has created work for Playback Theatre, Barking Gecko Theatre, and the University of Western Australia. He has composed for choreographers Chrissie Parrott, John McLaughlin, Douglas Dunn and dance graduates from WAAPA. For Tasdance, Jonathan composed for Chrissie Parrott's work Swimming in the Lunar Sea in Tasdance's 2004 production Light and Shade.